U.S. patent number 4,266,135 [Application Number 05/973,570] was granted by the patent office on 1981-05-05 for method of determining collimator aperture efficiency and apparatus with an efficient collimator aperture size.
This patent grant is currently assigned to Ohio Nuclear, Inc.. Invention is credited to Arthur B. Braden, John Covic, John J. Kuwik, Samuel K. Taylor.
United States Patent |
4,266,135 |
Kuwik , et al. |
* May 5, 1981 |
**Please see images for:
( Certificate of Correction ) ** |
Method of determining collimator aperture efficiency and apparatus
with an efficient collimator aperture size
Abstract
The method of determining collimator aperture efficiency for a
computerized tomographic scanner having a plurality of radiation
detectors positioned along a detector arc, a source of radiation
having a focal spot having radiation flux distribution positioned
across a scan circle from at least part of the detector arc, and a
collimator having a plurality of apertures arranged in a collimator
arc around said source. Also disclosed is a tomographic scanner
having a circular detector arc, a radiation source moveable along a
circular arc and an arcuate collimator having apertures in which
the width of the collimator aperture in the geometry of the system
has high efficiency. The ratio of the detector radius to the source
radius, to the collimator radius, to the aperture width is
36:24.3;9.4:0.024.
Inventors: |
Kuwik; John J. (Hudson, OH),
Braden; Arthur B. (Solon, OH), Taylor; Samuel K.
(Chardon, OH), Covic; John (Wickliffe, OH) |
Assignee: |
Ohio Nuclear, Inc. (Solon,
OH)
|
[*] Notice: |
The portion of the term of this patent
subsequent to February 26, 1997 has been disclaimed. |
Family
ID: |
27123593 |
Appl.
No.: |
05/973,570 |
Filed: |
December 27, 1978 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
812317 |
Jul 1, 1977 |
4190773 |
|
|
|
Current U.S.
Class: |
378/16; 378/147;
976/DIG.444 |
Current CPC
Class: |
A61B
6/032 (20130101); G21K 5/10 (20130101); A61B
6/4275 (20130101); A61B 6/06 (20130101) |
Current International
Class: |
A61B
6/03 (20060101); A61B 6/06 (20060101); G21K
5/10 (20060101); A61B (); G01N 023/08 () |
Field of
Search: |
;250/445T,360,505,509,514 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Smith; Alfred E.
Assistant Examiner: Grigsby; T. N.
Attorney, Agent or Firm: Fay & Sharpe
Parent Case Text
This application is a continuation-in-part of application Ser. No.
812,317, filed July 1, 1977 entitled "Shutter for Rotary Sources CT
Scanners", now U.S. Pat No. 4,190,773.
Claims
We claim:
1. A radiation aperture system for a CT scanner having a rotating
source of radiation and a detector array; a series of stationary
radiation detectors partially encircling the path of the source in
the same plane at less than 1.degree. intervals, the radiation
aperture system comprising:
collimator means for restricting the radiation to an energy
distribution having alternating relatively high energy and
relatively low energy regions, said collimator means being mounted
for rotation about the source; and,
drive means for causing counter rotation of said collimator means
about the source as the source rotates such that each one of said
relatively high energy regions is trained continously generally on
a single stationary detector during part of rotation of the
source.
2. The system of claim 1 wherein the radiation detectors are at
generally 0.5.degree. intervals.
3. The system as set forth in claim 2 in which said collimator
means includes a plurality of apertures, the number of collimator
apertures being substantially equal to the number of stationary
detectors.
4. The system as set forth in claim 3 wherein the ratio of the
radius of the source path to the width of each collimator aperture
is generally 1000:1.
5. The system as set forth in claim 3 wherein the ratio of the
radius of the source path to the width of each collimator aperture
is 24.3:0.024.
6. The system as set forth in claim 3 wherein the ratio of the
radius of the detector array to the width of each collimator
aperture is substantially 36:0.024.
7. The system as set forth in claim 3 wherein said collimator
apertures are arranged along a generally circular arc and the ratio
of the radius of the detector array to the radius of the source
path to the radius of the collimator aperture arrangement to the
width of each collimator aperture is substantially
36:24.3:9.4:0.024.
8. The system as set forth in claim 3 wherein the width of each
collimator aperture is substantially 0.024 inches.
9. A computerized tomographic scanner comprising:
a generally circular detector arc along which a plurality of
radiation detectors are able to be disposed, the radius of the
detector arc being the detector radius;
a radiation source rotatable along a generally circular arc, the
radius of the arc being the source radius;
a collimator comprising an arcuate member interposed between the
source and at least some of said detectors, said arcuate member
having a plurality of apertures each aperture having a width, the
radius of the arcuate member being the collimator radius;
wherein the improvement comprises the ratio of said source radius
to said collimator radius to said collimator aperture width being
substantially 24.3:9.4:0.024.
10. The scanner as set forth in claim 9 wherein said radiation
detectors are stationarily arranged along the detector arc at
0.5.degree. center-to-center spacings and said collimator apertures
are arranged along the arcuate member at 0.3.degree.
center-to-center spacings.
11. The scanner as set forth in claim 9 wherein the ratio of said
detector radius to said collimator aperature width is generally
36:0.024.
12. The scanner as set forth in claim 11 wherein said collimator
aperture width is generally 0.024 inches.
13. In a computerized tomographic scanner having a plurality of
radiation detectors positioned along a detector arc, a source of
radiation having a focal spot which has a radiation flux
distribution, said source being positioned across a scan circle
from at least part of the detector arc, and a collimator defining a
plurality of apertures arranged in a collimator arc around said
source, each aperture having a width in its dimension along the
collimator arc, a method of fashioning an efficient collimator
aperture width comprising:
(a) determining a first intercept of a first cutting line through
the focal spot and a line projected from a point on the detector
arc tangent to a first corner of a first aperture of said
collimator;
(b) determining a second intercept of said first cutting line and a
line projected from said point on the detector arc tangent to a
second corner of said first aperture;
(c) summing the radiation flux of the flux distribution along at
least that part of the first cutting line within the boundary of
the focal spot and between said first and second intercepts to
determine an amount of flux impinging on said point;
(d) repeating steps (a), (b), and (c) for another point on the
detector arc;
(e) repeating step (d) for points along the detector arc for at
least half the distance of the center-to-center spacing of a pair
of adjacent detectors;
(f) summing the flux impinging on said points along an arc length
at least half the length of a detector;
(g) summing the flux impinging on said points on said detector arc
and passing through said first aperture;
(h) finding the ratio of the sum of step (f) to the sum of step (g)
to obtain the collimator aperture efficiency;
(i) repeating steps (a) through (h) for each of a plurality of
collimator aperture widths; and
(j) fashioning the collimator aperture in conformity with the width
of the plurality of collimator widths which has the highest
collimator aperture efficiency as obtained in steps (a) through
(i).
14. The method as set forth in claim 13 further including repeating
steps (a), (b) and (c) for at least a second cutting line through
the focal spot and summing the flux from the flux distribution of
the first and at least the second lines.
15. The method as set forth in claim 13 further including between
steps (b) and (c);
determining a third intercept representing the intersection of said
first cutting line and a line projected from said point on the
detector arc through a third corner of said first aperture;
determining a fourth intercept representing the intersection of
said first cutting line and a line projected from said point on the
detector arc through a fourth corner of said first aperture;
discarding two of said first, second, third and fourth intercepts
whereby step (c) becomes summing the flux along the flux
distribution of the first cutting line between the two remaining
intercepts.
16. The method as set forth in claim 15 wherein the lines
corresponding to the two discarded intercepts intersect said
collimator and said two remaining lines traverse said aperture.
17. The method as set forth in claim 13 wherein said tomographic
scanner includes a patient scan circle and said source of radiation
is a fan of radiation sufficient to span said patent scan circle,
said method further including repeating steps (a) through (h) for
an aperture positioned in a plurality of positions around the focal
spot between the center of said fan and at least one edge of said
fan.
18. The method as set forth in claim 13 wherein said flux
distribution is ##EQU8## wherein ##EQU9## wherein
and w is the width of said focal spot.
19. The method as set forth in claim 13 wherein said flux
distribution is generally ##STR1## where K is a constant and where
w is the width of the focal spot.
20. A radiographic scanner comprising:
a plurality of radiation detectors disposed in a spaced
relationship along a detector arc at center to center intervals of
less than 1 degree;
a source of radiation which has a focal spot and a radiation flux
distribution across the focal spot, the source being disposed
across a scan circle from at least a part of the detector arc to
irradiate at least a part of the detector arc;
an arcuate collimator disposed around the source in a collimator
arc, the collimator defining a plurality of generally rectangular
collimator apertures at least some of said apertures being disposed
substantially in the collimator arc between the focal spot and the
detector arc, each aperture having a width which is substantially
parallel with the collimator arc, the width of each aperture being
selected by determining the efficiency of a plurality of aperture
widths and selecting the aperture width which has the highest
efficiency of said plurality of aperture widths; and
drive means for rotating said collimator around the source such
that each collimator aperture remains disposed between the source
and a corresponding one of the radiation detectors during rotation
of the source and the collimator, whereby the collimator defines a
plurality of collimated beams of radiation each of which is trained
on a corresponding detector during rotation.
21. The scanner as set forth in claim 20 wherein said efficiency is
determined by:
(a) determining the amount of radiation impinging on a segment of
the detector arc, which segment has a length which is at least one
half the center to center interval of the detectors;
(b) determining the amount of radiation impinging upon a part of
said segment which corresponds to at least one half a radiation
detector; and
(c) determining a ratio of the amount of radiation which impinges
on said part of said segment to the amount of radiation which
impinges on said segment, said efficiency being proportional to
said ratio.
22. The scanner as set forth in claim 20 wherein said efficiency is
determined by:
(a) determining a first intercept representing the intersection of
a first cutting line through the focal spot and a line projected
from a point on the detector arc tangent to a first corner of a
first aperture of said collimator;
(b) determining a second intercept representing the intersection of
said first cutting line and a line projected from said point on the
detector arc tangent to a second corner of said first aperture;
(c) summing the radiation flux of the flux distribution along at
least that part of the first cutting line within the boundary of
the focal spot and between said first and second intercepts to
determine an amount of flux impinging on said point;
(d) repeating steps (a), (b), and (c) for another point on the
detector arc;
(e) repeating step (d) for points along the detector arc for at
least half the distance of the center-to-center spacing of a pair
of adjacent detectors;
(f) summing the flux impinging on said points along an arch length
at least half the length of a detector;
(g) summing the flux impinging on said points on said detector arc
and passing through said first aperture; and,
(h) finding the ratio of the sum of step (f) to the sum of step (g)
to obtain the collimator aperture efficiency.
23. The scanner as set forth in claim 22 wherein said efficiency is
further determined by between steps (b) and (c);
determining a third intercept representing the intersection of said
first cutting line and a line projected from said point on the
detector arc through a third corner of said first aperture;
determining a fourth intercept representing the intersection of
said first cutting line and a line projected from said point on the
detector arc through a fourth corner of said first aperture;
discarding two of said first, second, third and fourth intercepts,
whereby step (c) becomes summing the flux along the flux
distribution of the first cutting line between the two remaining
intercepts.
24. A radiographic scanner comprising:
a plurality of radiation detectors disposed in a spaced
relationship along a detector arc at center-to-center intervals of
1 degree or less;
a source of radiation which has a focal spot and a radiation flux
distribution across the focal spot, the source being disposed
across a scan circle from at least a part of the detector arc to
irradiate at least the part of the detector arc; and
an arcuate collimator disposed around the source in a collimator
arc, the collimator defining a plurality of generally rectangular
collimator apertures, at least some of said apertures being
disposed substantially in the collimator arc between the focal spot
and the detector arc, each aperture having a width which is
substantially parallel with the collimator arc, the width of each
aperture being dimensioned such that a ratio of (a) the total
radiation flux impinging on the part of the detector arc to (b) the
total radiation flux which impinges upon the radiation detectors
along the part of the detector arc is 1 to at least 0.85.
25. The scanner as set forth in claim 24 wherein said ratio is
determined by:
(a) determining a first intercept representing the intersection of
a first cutting line through the focal spot and a line projected
from a point on the detector arc tangent to a first corner of a
first aperture of said collimator;
(b) determining a second intercept representing the intersection of
the first cutting line through the focal spot and a line projected
from the point on the detector arc tangent to a second corner of
the first aperture of said collimator;
(c) summing the radiation flux of the flux distribution along at
least that part of the first cutting line within the boundary of
the focal spot and between said first and second intercepts to
determine an amount of flux impinging on said point;
(d) repeating steps (a), (b), and (c) for another point on the
detector arc;
(e) repeating step (d) for points along the detector arc for at
least half the distance of the center-to-center spacing of a pair
of adjacent detectors;
(f) summing the flux impinging on said points along an arc length
at least half the length of a detector;
(g) summing the flux impinging on said points on said detector arc
and passing through said first aperture; and
(h) determining the ratio of the sum of (f) to the sum of (g).
Description
BACKGROUND OF THE INVENTION
The invention relates generally to the field of radiation imaging
of internal structures and, more specifically, to computerized
transaxial tomographic (CT) X-ray scanners. Unlike conventional
exposed film X-ray apparatus, the CT scanner produces narrow beams
of radiation, either X-ray or gamma rays, through plural coplanar
paths defining a cross-sectional or tomographic view of the
patient's internal organs, such as the brain. The attenuated beams
are sensed by radiation detectors whose electrical output is
indicative of the intensity of the radiation received by the
detector. One of the early types of CT scanners referred to in the
patent literature is shown, for example, in Hounsfield U.S. Pat.
No. 3,778,614. This system is generally referred to in the art as
the "translate and rotate" system. A source and a single detector,
for example, are aligned opposite each other on a mechanism which
causes the beam path between the source and detector to move
laterally across the scan circle. After rotating the
source/detector carriage assembly to a new orientation, the
translational scan is repeated. Readings are taken at uniformly
spaced parallel beam locations and representative values are
digitally stored. Data from a full set of scans involving numerous
relocations of the beam path is manipulated according to known
mathematics involving "back projection" to arrive at a digital
representation of the tomographic image. This digital
representation is converted to a tomogram which can be viewed on a
cathode ray tube. Ohio-Nuclear, Inc. markets a type of translate
and rotate CT scanner under the trademark "DELTA SCAN".
The major disadvantage of the translate and rotate system is
slowness of the scan mechanism due to the different alternating
types of motion. The major advantages of the translate and rotate
system are due to the fact that a single detector scans across the
entire scan circle thus enabling sampling at any time and avoiding
the need to have matched detectors or gain matching.
Another type of scan technique called "purely rotational" employs a
fan beam source with a subtended detector array in a fixed
relationship such that the fan beam and detector array rotate with
each other. This system has a major disadvantage. Numerous
detectors are required and none scans across the entire patient.
Thus, the sampling resolution is lowered and gain matching of the
detectors is required. The major advantage of the purely rotational
system is its high scanning speed. The high speed of the scanning
motion is desirable to avoid the effect on the image of the
resultant displacement of organs due to a patient's breathing.
It has been found that computer image reconstruction can be
accomplished with yet another arrangement of source and detectors.
In this new system, the detector array is a stationary arc of
uniformly spaced detectors about the center point in the scan
circle. The fan pattern source revolves about the center point
inside the detector array irradiating the scan circle and
subtending at any given time only a fraction of the detectors in
the total array. If desired, the array may be a complete circle or
ring. The reconstruction algorithms are described in
Lakshminarayanan, "Reconstruction from Divergent Ray Data",
Technical Report No. 92, State University of New York at Buffalo,
Computer Sciences Department, January, 1975 and copendent
application Ser. No. 838,089, filed Sept. 30, 1977 now U.S. Pat.
No. 4,170,835
The new type of scanning system, although requiring numerous
detectors and somewhat more elaborate digital processing for
reconstructing an image, provides the advantage of high scanning
speed due to the single mechanical motion for rotation while also
providing the capability of achieving high sampling resolution and
avoiding gain matching requirements because each detector views the
source across the entire scan circle.
If the circular array of detectors does not fully encircle the
patient, it is possible for the patient to be exposed to unused
radiation when the source approaches the terminus of its orbit and
part of the fan pattern falls outside the detector array. Another
problem is presented when the detectors are spaced apart throughout
the array since the fan pattern is not aligned with specific
detectors but instead floods the scan circle. In this case, a
portion of the radiation falls between adjacent detectors and is
not used for data collection. This radiation dosage is received by
the patient, however, even though it is not used.
An excessive radiation dose problem is presented even when a
collimator is used to divide the fan pattern of radiation into
finger beams. Because a rotating anode X-ray tube is not a true
point or line source of radiation, even a narrow collimator slit
will not focus all the radiation on a detector. Rather some of the
radiation dosage received by the patient fails to fall within
active detector limits.
SUMMARY OF THE INVENTION
The purpose of the invention is to reduce the dosage of unused
radiation which the patient receives when a rotating source is used
with a series of stationary detectors in a CT scanner system. This
is accomplished by employing an eclipsing shutter mechanism to
limit the portion of the fan pattern of radiation passing through
the scan circle at all times to a width coincident with the
subtended portion of the detector array. When the detectors are
sufficiently spaced apart in the detector array, unnecessary dosage
is reduced by dividing the fan pattern into a plurality of discrete
diverging beams and keeping them trained on respective detectors
for as long as they are within the scan circle. Unnecessary dosage
is further reduced by optimizing the collimator aperture width.
The CT scanner arrangement to which the invention applies is one in
which the source rotates and a series of detectors is spaced about
the center of rotation coplanar with the orbit of the source. A
radiation shield restricts the radiation from the source to a solid
fan pattern centered on the axis of rotation. An eclipsing shutter
mechanism about the source restricts the fan pattern at all times
to a pattern which will fall on the detector array as the source
traverses its orbital path. The shutter mechanism may include a
single aperture for flooding the scan circle or a multi-apertured
collimator, with one aperture for each detector, for training each
one of the discrete beams collectively defining the fan pattern on
a specific detector for as long as each beam intersects the scan
circle. The shutter mechanism is responsive to rotation of the
source which causes a specific fractional amount of rotation in the
opposite direction as the source moves. When the collimator is
employed the respective apertures keep themselves aligned between
the source and their respective detectors through the scan circle.
In the preferred embodiment, the means for rotating the shutter
mechanism is an epicyclic gear train, although other means are
possible such as a d.c. motor servo drive, or the like.
Another aspect of the invention is a method of determining the
aperture or slit efficiency for collimator slit widths. The slit
efficiency or ratio of radiation flux within active detector limits
to the total radiation flux passing through a collimator aperture
may be used to minimize the unused radiation dosage distributed
outside the detector limits.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of CT scanner apparatus associated
with the invention;
FIG. 2 is a plan view of the gantry with portions broken away to
expose the epicyclic gear train driving the shutter mechanism;
FIG. 3 is a side schematic detail view of the epicyclic gear
train;
FIGS. 4, 5 and 6 are schematic representations of the relative
positions of the source, shutter mechanism, resulting fan pattern
and detectors at three different orientations;
FIG. 7 is a plan view, similar to FIG. 2, showing a shutter
mechanism having a multi-apertured collimator and the resulting
discrete, diverging beams of the fan pattern;
FIGS. 8, 9 and 10 are schematic representations of the relative
positions of the source, collimated beams and detectors at three
different orientations;
FIG. 11 is a schematic diagram of a CT scanner with collimator
showing projection lines;
FIG. 12 is a typical radiation intensity distribution along a
detector arc segment;
FIG. 13 is an illustration of a preferred model for radiation flux
distribution across the focal spot of a rotating anode X-ray tube;
and
FIG. 14 is a section view of the preferred model of flux
distribution of FIG. 13 along the X axis.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 illustrates the mechanical apparatus associated with the
rotation source type CT scanner system. A gantry assembly 10
includes a U-shaped frame 12 pivotally supporting a gantry 14
having a central circular opening 16 through which a patient is
inserted for a body scan, for example, on a two-piece patient table
18. Shown in phantom, the source 20 produces radiation in a
coplanar fan pattern directed towards the opposite side of the
opening 16 and intersecting the center of the opening 16.
Mechanisms within the gantry 14 rotate the source 20 clockwise
about an axis through the center of the opening 16 perpendicular to
the fan pattern. A ring of detectors 22, also shown in phantom in
FIG. 1 is disposed within the gantry 14 concentrically to the
opening 16 and at a somewhat greater radius from the center of the
opening 16 than the source 20. The detector ring 22 lies in the
same plane as the fan pattern. The signals produced by detectors
which are within the fan pattern are applied to a number of
respective signal processing channels. By using the multiplexing
system described in the copending application Ser. No. 783,732,
entitled "Data Multiplexing System for CT Scanner with a Rotating
Source", filed Apr. 1, 1977 and assigned to the assignee of the
present application, the number of signal processing channels can
be reduced to the maximum number of detectors subtended by the fan
pattern and the detectors can time share these signal processing
channels. The copending application is incorporated by reference
herein.
In FIG. 2, the view of the gantry 14 shows the source 20 at a low
point on its orbit through circular path 20a. The source 20 lines
within a radiation shield 24 having a sector missing which causes
the radiation directed toward the opening 16 to assume a fan shape.
The thickness of the fan in the direction orthogonal to the paper
is slightly divergent and at the center of rotation represents the
thickness of the slice or tomogram to be reconstructed. Between the
source 20 and the opening 16 a shutter mechanism 26 having either a
single aperture or a series of very closely spaced apertures
concentric to the source 20 is mounted for rotation on a planetary
gear 28 rotatable upon an axis coinciding with the source 20.
The source 20 with shield 24 produces a fan pattern of radiation
whose angular width determines the diameter of a patient scan
circle 30 at a given distance from the center of the scan circle.
The scan circle 30 includes the area common to the fan at different
positions of the source 20 along its orbit 20a. The area within the
scan circle is the area which the reconstructed image will
represent. Thus, this area will coincide with the examined portion
of the patient's body, for example, the head.
The angular size of the single aperture 26a (FIGS. 4-6) in the
shutter mechanism 26 is dependent upon the number and spacing of
detectors 22. However, the arc spanned by the aperture 26a in the
shutter mechanism is less than the arc spanned by the array of
detectors 22. For example, if there are 424 detectors with
half-degree spacing from the center line of one detector to the
center line of the next detector, they cover an arc of 211.degree.
on the detector ring 22a. In the embodiment illustrated in FIGS.
2-6, the resulting arc spanned by aperture 26a in the shutter
mechanism is only 127.2.degree., i.e., 60% of the arc spanned by
the detector array.
With reference to FIGS. 2 and 3, a compound gear 32 is used to
drive the planetary gear 28 on which the source 20, fan shield 24
and shutter 26 are arranged. Compound gear 32 includes a smaller
spur gear 34 driving the planetary gear 28 and a larger spur gear
36, connected for rotation with smaller gear 34, engaging a
stationary ring-shaped sun gear 38 affixed to the gantry and
concentric with the scan circle about center c. As the source 20
orbits in a clockwise direction along its path 20a, the compound
drive gear 32 is caused to rotate clockwise which in turn causes
the planetary gear 28 and shutter 26 to rotate counterclockwise. As
shown in FIG. 3 in schematic form the axes of the gears are all
fixed with respect to each other. For purposes of illustration,
this is indicated by arm 40 of FIG. 3 to which the imaginary axle A
of the sun gear 38 coinciding with the center c of the scan circle
is journalled along with the common axle of the compound gear 32
and the axle of the planetary gear 28. Since the sun gear is fixed,
the arm 40 is free to rotate about the imaginary axle A. This
rotation produces the orbit of the source 20 with shield 24. This
motion is analogous to a solar, planet and moon system in which the
sun gear 38 represents the sun, the source 20 represents the earth
and a fixed point on the planetary gear 28, for instance, the
aperture in the shutter mechanism 26 represents the moon.
The requirement for this type of motion between the shutter
mechanism and the source is demonstrated in FIGS. 4, 5 and 6
showing progressive clockwise orientations of the source. In FIG.
4, the source is shown at the initial point S.sub.i of the scan
cycle. In FIGS. 5 and 6, the source is shown at the midpoint
S.sub.m of the scan cycle and at the final or end-point S.sub.f of
the scan cycle, respectively. FIGS. 4, 5 and 6 also show detector
ring 22 and, for illustration, the first, last and three
intermediate stationary detectors D1, D2, D3, D4 and D5 in the
ring. Of course, in the practical embodiment there are many
detectors in the spaces between detectors D1-D5. The arc 22a is
bounded by detectors D1 and D5 and defines the detector array span.
Arc 22b is that portion of the detector ring which is outside the
detector array.
In FIG. 5, the source 20, with shield 24, produces an X-ray field
defined by the fan pattern 20b having outer, diverging boundaries
B1 and B3 which define the included angle of the fan pattern which
floods the scan circle 30. As shown in FIGS. 4 and 6, the eclipsing
shutter mechanism 26 acts to reduce the included angle of fan
pattern 20b whenever the source approaches either terminus S.sub.i
or S.sub.f of its cycle. This is desirable because the array of
detectors on ring 22 does not entirely encircle the scan circle and
if the width of the radiation field was not so restricted, unused
radiation would pass through scan circle 30 and needlessly increase
the patient dosage.
The eclipsing effect of the shutter 26 is accomplished by utilizing
the epicyclic mechanism of FIG. 3. It should, of course, be
understood that other drive mechanisms such as, by way of example,
a d.c. motor servo drive may also be employed. In the illustrated
embodiment, the included angle between the first detector D1 and
the last detector D5 is 212.degree.. The remaining arc 22b of ring
22, 148.degree. in the example, is outside the detector array. The
shutter mechanism 26 is utilized to preclude the projection of any
portion of the X-ray of fan pattern 20b through the scan circle if
that portion of the field would fall outside the detector array,
i.e., on arc 22b.
With the arrangement shown in the drawings, a 60% rotation of
shutter 26 is required for each 100% positive rotation of source 20
about its orbital path 20a. The source moves from point S.sub.i to
point S.sub.f on path 20a in the direction of arrow S during a scan
cycle, and the shutter rotates in the opposite or negative
direction as indicated by arrow T. As shown in FIG. 4, the leading
edge 26b of the shutter aperture is on a straight line projecting
from detector D1 to source 20 for as long as a straight line
projecting from detector D1 to the source intersects the scan
circle 30. Thus, the shutter imposed boundary B.sub.L of fan
pattern 20b is trained on detector D1 continuously for as long as
the detector is in the data-taking portion of the scan cycle. There
may be some minor deviation of the imposed boundary B.sub.L, which
is in effect the same as the deviation of an individual slit in the
rotating collimator described below. The portion of the fan pattern
between the trailing edge B1 of the fan pattern and the shutter
imposed boundary B.sub.L is blocked by the shielding portion 26d of
shutter 26 and does not pass through the scan circle 30. This is
desirable since this portion of the fan pattern would fall outside
the detector array and would needlessly increase the patient's
exposure to radiation.
As the source traverses about its orbital path 20a in the direction
of arrow S (clockwise), the shutter 26 rotates in the direction
indicated by arrow T (counterclockwise) 0.6.degree. for every
1.degree. of source rotation, ever increasing the fan pattern width
while continuously training the shutter imposed boundary B.sub.L of
the fan beam on the detector D1 for as long as the detector D1 is
in the data-taking portion of the cycle, i.e., the scan circle is
intermediate the detector D1 and the source 20. The fan pattern
continues to widen until the entire fan pattern bounded by leading
edge B3 and trailing edge B1 falls on the detector array. At this
point, the shutter aperture is completely out of the path of the
fan pattern emanating from source 20 and the entire fan pattern
floods the patient scan circle 30. This is desirable since the
entire fan pattern falls on the detector array.
The source 20 is shown at the mid-point S.sub.m of its travel in
FIG. 5. This is representative of the flooded scan circle wherein
the leading edge B3 of the fan pattern 20B falls on detector D2 and
the trailing edge B1 falls on detector D4. As the source 20
continues its movement, leading edge B3 of the fan pattern
approaches detector D5, the last detector in the array. Again, it
is desirable to block any portion of the fan pattern which will
fall outside the detector array span. At this point, illustrated in
FIG. 6, trailing edge 26c of the shutter aperture has rotated into
blocking relationship with the source and shields the leading edge
B3 and a portion of the fan pattern 20b, training the shutter
imposed boundary B.sub.K on the last detector D5 for as long as the
detector D5 is in the data-taking portion of the scan cycle. After
the source has completed its orbital cycle by traversing to point
S.sub.f, both the source and the shutter are returned to the
initial position S.sub.i of FIG. 4.
By utilizing the shutter mechanism 26, the source can be rotated
through any portion of its orbital path without passing any
radiation through the scan circle that does not ultimately fall on
the detector array span. The single apertured shutter mechanism 26
of FIGS. 1-6 is practical whenever the detectors in the detector
array are spaced in such a manner that it is desirable to flood the
scan circle with radiation.
It is possible to further reduce the patient dosage by subdividing
the fan pattern into a plurality of discrete beams, each trained on
a particular detector for as long as the detector is in the
data-taking portion of the scan cycle. In another embodiment of the
present invention, this beam subdivision is illustrated for a
system in which the center lines of adjacent detectors 22 are
spaced apart approximately one half degree about the detector
ring.
The beam subdivision is accomplished by replacing the
single-apertured shutter 26 with a multi-apertured shutter or
collimator 126 as shown in FIGS. 7-10. The first and last apertures
in the rotating collimator effectively perform the same function as
the trailing and leading edge of the eclipsing shutter. The number
of apertures in the collimator 126 corresponds to the number of
detectors 22. However, as with the single-apertured shutter 26, the
arc spanned by the apertures in the collimator 126 is less than the
arc spanned by the detectors 22. For example, if there are 424
detectors with 1/2.degree. spacing from the center line of one
detector to the center line of the next detector, they cover an arc
of 212.degree. on detector ring 22a. There would be 424 closely
machined apertures in the collimator 126, one for each detector.
However, as with the shutter aperture 26a, the arc spanned by the
424 apertures is again only 127.2.degree., i.e., 60% of the arc
spanned by the detector array in the geometry of the embodiment
illustrated.
The apertures in the collimator 126 make a plurality of narrow
discrete diverging equally, angularly spaced beams which
collectively form the fan pattern 20b whose angular width at a
given distance from a scan center determines the diameter of the
patient's scan circle 30. The object of the collimator 126 is to
keep each beam of fan pattern 20b aimed toward a specific single
detector while the source 20 moves along the path 20a. Of course,
this is only of importance while the line between the particular
detector and the source intersects the scan circle 30. The line
will intersect the scan circle over the angle defined as the
"detector angle" subtended from the detector by the diameter of the
scan circle.
The epicyclic motion between the collimator ring and the source is
demonstrated in FIGS. 8, 9 and 10 showing progressive clockwise
orientations of the source separated by 40.degree.. FIG. 8 shows
three successive source positions S1, S2 and S3 each separated by
40.degree. as the source traverses orbit 20a in the clockwise
direction. FIG. 8 also shows detector ring 22 and, for
illustration, five stationary detectors D10, D20, D30, D40 and D50.
Source 20 produces a fan of plural diverging beams centered
collectively on the center c of rotation of the source 20. Three of
these plural beams B1, B2 and B3 have been selected for FIG. 8
because of their geometric significance. As in FIGS. 4-6, beams B1
and B3 are the peripheral beams of the fan beam pattern and are
tangent to the scan circle 30. Beam B2 passes through the center c.
The fan beam width is twice .alpha., where .alpha. is the angle
made by beams B2 and B3. In practice, there are many beams between
B1 and B2 and B2 and B3. Likewise, there are many detectors between
each one of the five detectors shown in FIG. 8. Detector D10 is
defined as the detector lying on the tangent from the source
position S1. Detector D20 is aligned with the center and position
S1 and detector D30 lines on the other tangent to the scan circle
from position S1.
In FIG. 8, beam B2 is defined by aperture A2, one of the apertures
in the collimator ring 26 of FIG. 2. In this orientation, the
aperture A2 sights the source along the center c. As the source 20
moves from position S1 to S2, it moves through the angle .phi.
about the center c of rotation. The radius r from the new source
position S2 to center c in FIG. 8 indicates the direction in which
the aperture A2 would aim the source if aperture A2 were fixed to
the source and thus always looking toward the center c. This would
mean that beam B2 would no longer be looking at detector D20 but
would be looking in the direction of detector D30. While the source
is moving from position S1 to S2, it is not desirable for the beam
B2 to move its target along the path from D20 to D30. Instead, beam
B2 should be continuously trained on detector D20 as shown in FIG.
9. This requirement dictates that aperture A2 will not be aligned
with the radius but rotate through the angle .alpha. to a new
angular orientation about the source 20 such that the beam B2
remains directed at detector D20. While the source is moving from
S1 to S2 through angle .phi., the aperture A2 must be moving
continuously through an angle which grows to angle .alpha. at the
same time. This means that the aperture A2 has to rotate
counterclockwise about the source 20 as the source 20 moves
clockwise from position S1 to position S2 in order to keep beam B2
trained on detector D20.
Aperture A2 represents but one of the apertures for the many
detectors between detector position D10 and detector position D30
in the practical embodiment. All of the beams have to slued or
scanned across the scan circle in the same manner as B2. For
example, beam B3 has just come into being, that is, has just
intersected the scan circle 30 and for the first time detector D30
is illuminated by radiation from B3. As the source 20 moves from
position S1 to S2, beam B3 remains trained on the detector D30 as
shown in FIG. 9. The only way that this can be accomplished is for
its corresponding aperture A3 to rotate the same amount and sense
or direction as aperture A2. Thus, in FIG. 9, beam B3 views
detector D30 through the center c although before in FIG. 8, beam
B3 started out on the periphery of the scan circle 30. At the time
shown in FIG. 8 detector D10 is at the point where it is about to
lose communication with the radiation fan altogether. Beam B1 has
been slued all the way across the scan circle 30 to its periphery
and as the source begins to move clockwise from position S1 even
slightly, the beam B1 trained on detector D10 by corresponding
rotating aperture A3 falls outside of the scan circle 30 and is
darkened by the fan shield 24 shown in FIG. 2. Thus, the beam B1
shown in FIG. 9 after the source has moved to position S2 is only
imaginary since it is blocked by the shield 24. The only active
beams in FIG. 9 are those beams B2, B3 and new beam B4 which has
just become tangent for the first time with the scan circle 30.
Aperture A4 trains beam B4 on detector D40.
In FIG. 10, after the source 20 has moved through another
40.degree. clockwise to position S3, beams B1 and B2 are no longer
in use and new beam B5 trained on detector D50 through aperture A5
has come into view, tangent to the scan circle 30. By the time
shown in FIG. 10, beam B3, which came into view for the first time
in position S1 in FIG. 8, has been slued through the center c of
the scan circle 30 in FIG. 9 and is now tangent for the last time
to the scan circle 30 just before it will pass out of view. The
aperture A2 which was originally centered on the center c in FIG. 8
has by the time the source has moved through 80.degree. in FIG. 10
moved around the source S3 so far that it is no longer even
sighting the source through the scan circle.
It is important to note that the angle through which aperture A2
has moved is not 80.degree.; it is less than 80.degree. because of
the geometry of the source and detector arrangement. The ratio of
the angular velocity of the source to the angular velocity of the
aperture A2 or any other aperture on the collimator (or the single
aperture 26a of FIGS. 1-6) is determined by the relationship
between the radius of the source orbit and the radius of the
detector ring. It can be shown that there are values for these two
radii at which the ratio of the angular velocities of the source
about the center c and the collimator ring (or shutter 26 in FIGS.
1-6) about the source 20 are rational numbers which can be
accommodated by a reduction gear train. For example, it can be
shown that if the detector ring has a radius of 36" and the source
has a radius of 24.3", the angular velocity of the collimator (or
shutter in FIGS. 1-6) around the source 20 should be exactly 60%
(in the opposite direction) of the angular velocity of the source
20 about the center c. This ratio (-0.60) is relatively amenable to
a toothed gear train solution.
It can also be shown that if the detector ring is kept at 36" and
the source is moved from 24.3 to 24.0" that the angular velocity
ratio becomes -0.603015. This ratio cannot be practicably resolved
with a simple gear train. Although it is true that a friction drive
of appropriately sized wheels might accommodate such a ratio, the
dimensional instabilities in a friction drive assembly make it
appear to be inappropriate for accurate registration of the
collimator assembly. Toothed gears on the other hand are extremely
accurate because there is no slippage allowed. This same
relationship holds true when the single apertured shutter 26 of
FIGS. 1-6 is utilized.
There is another anomalous factor at work in the geometry of FIGS.
8-10. Even with the "best" angular velocity ratio, namely 0.603015
for a source location of 24.0", the beams do not tract the
detectors perfectly, that is, there is a slight aberration or error
as the source orbits. The center line of a given beam, although
aimed in the direction of a particular detector, will travel
slightly over the center line of the detector.
There is another reason why the location of the source at 24.0"
would have been undesirable. A very minor change in the source
location from 24.0" to 24.3" not only caused the angular velocity
ratio to become a rational number (-0.6000) but also at the same
time it reduced the error of the beam tracking to within a range of
plus or minus 1.037 to 1.078 millimeters. This tracking linearity
error was calculated for the condition where the X-ray beam is
emitted through particular aperture over approximately 80.degree.
of source rotation, the full angle in FIG. 8 through which each
beam is slued through the scan circle 30.
These dimensions are, in fact, used for the preferred embodiment, a
commercial model of which is under development by the assignee.
Thus, with the source orbit at 24.3" and the center of each
detector face at 36.0", the ratio of -0.60 for the angular
velocities of the source and collimator or shutter is accomplished
by the following gears, referring to FIG. 3: the sun gear 38 has
600 teeth; the larger compound gear 36 engaging the sun gear 38 has
100 teeth; the smaller compound gear 34 engaging the planetary gear
28 has 40 teeth and the planetary gear 28 has 400 teeth. The ratio
of the number of teeth on the sun gear 38 to the number of teeth on
the larger compound gear 36 multiplied by the ratio of the number
of teeth on the smaller compound gear 34 to the number of teeth on
the planetary gear 28 is 0.600. Any other kind of epicyclic gear
train that reverses the sense of rotation of the planetary gear and
collimator ring 26, and provides a reduction of exactly 60% will
do. It also appears, because of the circular geometry of the
system, that the dimensions of the radii of the source and detector
ring will scale properly; that is, the ratio of the detector ring
radius to the source radius, 1.48, for the preferred embodiment,
will require an angular velocity ratio for the collimator and
source of exactly 60% no matter what the size of the system. With a
detector spacing of 1/2.degree. of collimator, aperture width of
0.60 mm, an outer collimator radius of 9.40 inches is preferred.
Other geometries of source diameter, detector diameter and
collimator diameter can be used provided the corresponding
adjustments to the gear ratios in the gear train are made.
FIG. 11 shows a schematic diagram of the geometry of a computerized
tomographic scanner to which reference is made in explaining the
method of determining collimator aperture efficiency. Briefly
stated the method entails determining the amount of radiation
impinging upon arc 22a and the amount of radiation impinging on
that portion of the arc that the detectors occupy. 100% efficiency
is achieved when all the radiation impinging on arc 22a impinges on
detectors and none in between.
The amount of radiation impinging on any point of the arc 22a is
affected by several factors. These include the geometry of the
system, especially the degree of alignment between the point, a
collimator aperture and the source, the size of the aperture and
the size and energy distribution of the source. The size
relationships of FIG. 11 have been distorted from the above
dimensions for clarity of representation of the principals of the
method. Position along arc 22a is described in terms of the angular
displacement .beta. along arc 22a.
As explained above, for each detector there is a specific aperture
in collimator 126 which remains positioned between the source 20
and the detector. Thus, for .beta. corresponding to the center of a
detector there is an aperture positioned about a line from the
center of the detector to the center of the source. For a point not
at the center of detector an aperture is at best only partially
aligned between the source and the point. For example, radiation
impinging upon a point 130 on arc 22a has passed through an
aperture in collimator 126 defined by points 132 and 134 on the
outer periphery of the collimator a distance RCO from the source
and by points 142 and 144 on the inner periphery of the collimator
a distance RCI from the source.
From the perspective of a detector, there are four readily
demarcatable paths which could mark the extremes of possible paths
between a point on a detector and the radiation source--the paths
from the detector through each of the four corners of the aperture.
These extreme paths are represented by four lines drawn from point
130 through points 132, 134, 142 and 144 which mark the corners of
the collimator aperture. Of course, only two of these four paths
can mark the actual extremes. Which of the lines through 132 and
142 and of the lines through 134 and 144 mark the boundaries varies
with the geometry and the angular displacement .beta. of point 130.
For any angular displacement .beta., one of the lines through 132
and 142 is tangent to the collimator at point 132 or 142, while the
other passes at least partially through the collimator structure. A
boundary path passing partially through the collimator is
physically impossible. Similarly, one of the paths through 134 and
144 is also always impossible. The two lines representing valid
extremes intercept the source 20, or more precisely, a focal spot
104 of the X-ray tube which forms the source.
These two lines representing the valid extremes delineate the
region of focal spot 104 which contributes to the total energy
received at point 130. The intensity of radiation flux, I, received
at point 130, may be represented mathematically as:
where f(x,y) is the energy distribution across the focal spot and
the limits of integration are defined by the lines representing the
valid extremes.
For simplicity of evaluation, the double integral may be
approximated as: ##EQU1## where f(x) is the energy distribution
along a line cutting the focal spot in the x direction, where
x.sub.1 and x.sub.2 are the intercepts of the valid extreme paths
with the cutting line and where the summation is over a plurality
of parallel cutting lines. If an intercept is beyond the boundary
of the focal spot, the integration is preformed only to the boundry
of the focal spot. Because f(x) is periodic in the preferred
embodiment, integration should not be carried out beyond the
boundary unless f(x) is set to zero in that ragion. The cutting
lines are a plurality of parallel imaginary lines across the focal
spot along which lines the energy distribution is known.
Any number of cutting lines through the focal spot may be used.
Equivalently, one cutting line may be viewed as being shifted
incrementally from one edge of focal spot 104 to the other. The
integral is evaluated between the intercepts of the valid extremes
on each cutting line. In the preferred embodiment, cutting line 150
assumes 8 positions providing 8 contributions to the energy
received at point 130. Further the energy distribution is
normalized, i.e. ##EQU2## where w is the width of the focal spot
along the x axis. Thus, the total amount of energy available from
along one cutting line of the focal spot is assigned, for each
computation, the arbitrary value of 1 unit of energy.
By stepping point 130 incrementally along arc 22a and recalculating
the intensity at each point along arc 22a a curve as exemplified by
curve 160 of FIG. 12 can be produced. After correlating the
radiation detectors 22 with the positions along 22a, the relative
amounts of radiation impinging upon and falling between the
detectors is determinable.
The efficiency of the collimator is, by definition, that percent of
the total radiation flux to which a patient is subjected which
impinges upon a detector. This efficiency is determinable from the
ratio of the amount of energy impinging upon that portion of arc
22a along which a detector is positioned to the amount of energy
impinging upon arc 22a generally. The shaded area 162 represents
the energy impinging on the portion of arc 22a which corresponds to
a detector. Note that area 162 is both under curve 160 and over a
detector 22. The area under one cycle of curve 160 represents the
energy impinging on arc 22a generally. In the illustration of FIG.
12, the area 162 is 33.89 units, and the area under one cycle is
39.06. For the amount of energy represented by curve 160, the
efficiency is 86.78%. With the open shutter of FIGS. 2, 4, 5, and
6, the amount of energy striking arc 22a is illustrated by curve
166.
The present method can be performed graphically. However, in the
preferred embodiment, a computer is programmed to carry out the
above analysis. Many programs and languages may be used. The
following steps provide one basis for implementation:
1. Assign x, y corrdinates to points 130, 132, 134, 142 and
144.
2. Project lines from point 130 through 132, 134, 142 and 144,
hereinafter line 130-132, line 130-134, line 130-142, and line
130-144 to intersect a first cutting line 150 through the focal
spot coordinates.
3. Determine the two valid boundary lines, e.g. determine the two
lines which have the inner most intercepts.
4. Perform the integration of f(x) between the intercepts of the
two valid lines with cutting line 150.
5. Shift the cutting line along the y axis one incremental step to
150'; calculate new intercepts and repeat the integration.
6. Repeat step 5 until the cutting line has been shifted along the
y axis a distance equivalent to the length of the focal spot.
7. Sum the products of the integrations.
8. Shift point 130 and repeat the above steps.
9. Calculate the ratio of the total flux impinging on points 130
along the detector arc corresponding to a detector to the total
flux passed by the collimator aperture and striking the detector
arc.
Alternately, in addition to or instead of step 9 a graphics
generating program may generate a print out similar to FIG. 12.
Calculation of the average efficiency over one scan can be achieved
by redefining the coordinates of points 130, 132, 134, 142 and 144
and the focal spot to simulate several positions within the
radiation fan. The efficiencies at positions spanning the fan or a
symeteric half of the fan are plotted or averaged.
By changing the coordinates of points 132, 134, 142 and 144 models
of different aperture widths, collimator thickness, collimator
sizes, etc. can be created allowing the efficiencies of different
collimator systems to be compared.
It will be appreciated that patient dose varies directly with the
collimator aperture width, i.e., halving the width halves the dose.
However, as seen from curve 160, detector flux does not vary
directly with collimator aperature width. In the above scanner
geometry, with a detector spacing on the order of 1/2.degree., a
collimator aperture of 0.60 mm in width reduces the patient dosage
to 50% of an uncollimated system; but the above analysis shows that
the average detector flux is reduced only to 75% of the
uncollimated system.
The energy distribution f(x,y) across the focal spot 104 is not
constant. For a rotating anode X-ray tube, microdensitometer
examination of the focal spot has shown that the X-rays are emitted
primarily along the edges of the focal spot. This has led others to
approximate the focal spot as a pair of parallel line sources.
Others still have developed such solutions as the dual filament
X-ray tube of U.S. Pat. No. 4,065,689 to make a more uniform focal
spot distribution.
In the present method, an energy distribution in the form of a pair
of parallel line sources has proven to be too inaccurate when
applied to very narrow collimator slits. For example, with the
above scanner geometry and detectors placed at 1/2.degree. of
detector arc spacings, a collimator does not appear practical with
the parallel line energy distribution.
On the other hand, the actual, physical energy distribution is so
complex that evaluation of the above double integral or line
integral of the present method is cumbersome.
To balance sufficient accuracy with ease of interpretation, the
energy distribution model of FIG. 13, shown in cross-section in
FIG. 14, is preferred. The energy distribution of FIG. 13 has a
base corresponding to the focal spot, which is canted 15.degree.
from the x, y plane. The amplitude along the y axis represents
energy. The distribution is generally in the form of slices through
a rounded trough. The cross-section 200 of this distribution is a
Fourier series expansion of curve 202 with a d.c. offset added for
normalization. To determine the Fourier transform, curve 202 is
approximated as a periodic function g(x) defined by:
______________________________________ g(x) = .645/w 0 .ltoreq. x
.ltoreq. .3w 1.5625/w .3w .ltoreq. x .ltoreq. .5w -1.5625/w .5w
.ltoreq. x .ltoreq. .7w -.625/w .7w .ltoreq. x .ltoreq.w
______________________________________
where w is the width of the focal spot. The Fourier expansion of g
(x) is ##EQU3## where ##EQU4## Selecting the Fourier series with
only the first two Euler coefficients, a.sub.1 and a.sub.3, as the
model of the energy distribution f(x) along the cutting line of the
focal spot, the mathematical model becomes ##EQU5## which is
illustrated as curve 204.
To further simplify the method, the total flux, I, from the energy
distribution across one cutting line is normalized, i.e., ##EQU6##
Thus, normalized, f(x) becomes: ##EQU7## which is shown graphically
as curve 200.
Other mathematical models, of course, can be used. For other types
of X-ray tubes, it is usually necessary to use other models.
However, for rotating anode X-ray tubes, the above model is an
exceptionally desirable compromise between accuracy and ease of
integration and is, accordingly, preferred.
This invention has been described with reference to the preferred
embodiments with some possible modifications thereto. Obviously,
other modifications and alterations will be obvious to others upon
the reading and understanding of this specification. It is our
intention to include all such modifications and alterations insofar
as they come within the scope of the appended claims or the
equivalents thereof.
* * * * *